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Wednesday, September 10, 2014

New Lundahl Interstage transformer for 845 amps


During my visit at Lundahl in Sweden, we discussed some new products which would be worthwhile to add to the line up. I suggested a 1:1 interstage transformer, capable of enough voltage swing to drive a 845 output tube and for use with powerful driver tubes which draw a lot of current.

Last week I received prototypes of the new LL2753 interstage transformer. It is capable to swing 330V peak to peak at 30Hz. Like most Lundahl products the air gap can be chosen for a wide range of bias currents. My sample pair came gapped for 20mA with a primary inductance of approximately 60Hy.

Initial measurements on the bench look very promising with a nice flat frequency response and a smooth roll off without severe resonance peaks.

The LL2753 comes on a larger core than the widely used LL1660. Here both of them in comparison:

Measurements in an actual tube circuit will be done soon. Stay tuned for more results. Also a SE 845 amp with these might be coming up in the near future.

Best regards



  1. Good news !
    Perfect for interstage with Elrog 845's :)
    Thanks for share Thomas.

  2. Could it be a good candidate for 211 tubes also?

  3. Do you think 20mA is enough current to drive the grid of 845?
    I never built high power SET but often heard people say that low
    driver current compromises the sound(not enough to overcome
    the miller capacitance of the power tube).

    1. Hi! 20mA is more than you will see in the drivers of most 845amplifiers. In case of the 845 the driver actually needs less current compared to a 211 since the 845 has a lower miller capacitance. But it needs higher drive voltage.


    2. Thanks Thomas.
      I'm very curious to see what you will do with this transformer.
      Looking forward for your posts.

  4. Hello Thomas,
    In the circuit in two stages, The LL2753 is right, then, well to the internal resistance of the valve 6hv5a? What value of input impedance and inductance have the version at 20ma?
    You should use the connection 1: 1 or you can configure a 2: 1 ratio (if possible), to a lower output impedance?
    Sorry for the many questions but before you order you would like transformers only a confirmation,
    Thanks again,


    1. Hi
      The LL2753 is not well suited for use with the 6HV5a.
      You can send an email if you want to order transformers, I can help to pick the right one

      Best regards